Hearing impaired individuals typically suffer from a loss of hearing that falls in one of two general categories: conductive and sensorineural. Conductive hearing loss results from a failure in the mechanical chain in the external and middle ear that captures and drives the sound to the cochlea. Sensorineural hearing loss is due to the deficiency or damage in the cochlea, particularly of the hair cells located in the cochlea, which converts the sound to electrical signals that are transmitted by the auditory nerve to the part of the brain that creates the sensation of hearing.
Conductive hearing losses may be corrected, at least partially by medical or surgical procedures or by using conventional hearing aids to amplify the sound in order to increase its energy and patient be able to perceive sounds of the external word. Sensorineural hearing loss on the other hand may be corrected using a cochlear stimulation systems or, cochlear implant.
Cochlear stimulation systems operate by converting sound to electrical signals, which are applied to the residual auditory system through an intracochlear electrode array. The intracochlear electrode array provides electrical stimulation directly to the auditory nerve fibers to create a sound perception in the brain of a patient using the cochlear stimulation system.
A typical cochlear stimulation system includes an audio pickup, or input (for example, a microphone), an amplifier, a sound processing system, and a receiver/stimulator coupled to an intracochlear electrode array. The intracochlear electrode array and receiver/stimulator are typically part of an implanted portion of the system. The audio pickup, amplifier and sound processor are part of the external components of a cochlear stimulation system. The audio pickup is typically located on an earpiece having a connection to the sound processing system. The sound processing system also connects, wirelessly or via a wire, to a transmitter that is typically attached to the patient's head near the audio pickup earpiece. The transmitter is attached to the head at a location that is closest to a receiver connected to the implanted portion. The transmitter typically communicates with the receiver via a magnetic coupling. The implanted portion includes electronics that is coupled to the intracochlear electrodes or intracochlear electrode array. The intracochlear electrodes extend and terminate sequentially in a straight or spiraling line. The intracochlear electrodes are inserted into the cochlear tissue along the spiraling line that follows the spiral formed by the structure of the cochlea.
The intracochlear electrodes are assigned frequency bands in the auditory frequency range in order from highest frequency bands to lowest such that the highest frequency band electrodes are processed closest to the electronics in the implanted portion; the lowest frequency bands are processed closest to the end of the spiraling line, near the apex, i.e., the conical tip of the cochlea. The ordering of the frequency bands conforms to the functional structure of the cochlea, which is known to process incoming sound representing the highest frequencies at the base, i.e., beginning of the cochlea's spiral shape. Low frequencies are processed by the cochlear tissue extending further into the spiral shape in descending order, such that the lowest frequencies are processed near the apex.
During operation of the cochlear stimulation system, the audio pickup receives sound input and transmits the electrical signals to the sound processing system. The sound processing system multiplexes the signal by filtering the signal at a bank of bandpass filters connected in parallel. Each bandpass filter in the bank of bandpass filters corresponds to a different one of the intracochlear electrodes. The filtered signal is then assigned a current simulation level, which corresponds to a current of the signal to be output at the corresponding intracochlear electrode. The current stimulation level delivered to the cochlea by each intracochlear electrode is adjusted hopefully according patient's loudness sensation. Assigning frequency bands and setting a current level to each intracochlear electrode allows the cochlear stimulation system to represent incoming sound signal into an activation sequence to the intracochlear electrodes selected according to a stimulation strategy programmed into the sound processing system (described below). Basically, the current stimulation level is selected from a voltage level or some other indicator of the sound intensity of the input sound signal.
The filtered signal at the assigned current stimulation level are then de-multiplexed and sent to the transmitter. The transmitter transmits the de-multiplexed signal using a magnetic coupling to the receiver in the implanted portion. The signal is multiplexed to extract the filtered signals and each filtered signal is coupled to the individual intracochlear electrode corresponding to the filtered signal's bandwidth. The filtered signals excite the nerve fibers at the location of the corresponding intracochlear electrodes at a current level that is intended to correspond to the sound intensity level of the input sound. The patient senses the sound as the combination of frequencies corresponding to the intracochlear electrodes that generated the filtered signal and the combination of sound intensities corresponding to the current levels at each intracochlear electrode.
When a patient is provided with a cochlear stimulation system, a surgical procedure is performed to implant the components referred to above as being part of the implanted portion inside the ear. During the procedure, the intracochlear electrodes are inserted into the cochlea, and the receiver is implanted in an area of the ear that is opposite a space where the transmitter may be placed. The patient is also provided with the transmitter and audio input connected to the sound processing system.
A few weeks after the implant procedure, the cochlear stimulation system is also “fitted” for operation. The purpose of fitting the cochlear stimulation system is to adjust the range of current stimulation levels for each intracochlear electrode. The adjustment is necessary to ensure that the minimum current stimulation levels correspond to the lowest possible threshold sound intensity level that the patient can hear, and a maximum current stimulation level that will not result in pain or discomfort at high sound levels. That is, fitting permits a physician to determine the minimum and maximum psychophysical values of the stimulation current for each intracochlear electrode.
Cochlear stimulation systems are programmed to use a minimum and a maximum current value that hopefully match the hearing threshold level and most comfortable loudness level of the patient. The current stimulation level typically refers to a minimum and a maximum value, depending on the specific cochlear stimulation system, i.e. the electric current dynamic range. The full range of current stimulation levels corresponds to a range of sound pressure levels (in dBHL) mapped according to the loudness perception of the patient. The sequence or order of activation of the intracochlear electrodes depends on the input sound features and stimulation strategy selected by the clinician, i.e., the code used to activate a subset of intracochlear electrodes according to the most important features of the incoming sound. The fitting of the implant involves generating a “MAP” of ranges of intracochlear electrode current stimulation levels, preferably meeting the particular needs of the patient. This means setting a threshold current stimulation level (or T level) and a maximum comfort level (or C level) for each electrode. Cochlear stimulation systems typically provide a procedure that allows a physician to set a T level and C level as well, to a desired value. It is assumed for purposes of this disclosure that the cochlear stimulation system being fitted provides such a facility, either using a manual mode that may be driven by software, or an automatic mode that permits downloading the T level from a computer or some other electronic device.
A variety of strategies exist for determining the T levels for each intracochlear electrode in a cochlear stimulation system. In some cases, the physician may choose to leave the cochlear stimulation system set to the T levels set by the manufacturer or use T levels in preconfigured maps of T levels to sound levels. The values of psychophysical parameters such as current stimulation levels are highly dependent on the physiology of the patient. Therefore, it is unlikely that predefined current stimulation levels would be suitable for many patients.
The physician may also use a subjective method where the physician stimulates the patient using a low level electrical current and increases the electric current level until the patient informs the physician that he can ‘hear’ the sound. The subjective method, however, cannot be implemented with children that cannot yet communicate. In fact, it is likely that any patient cannot communicate if they are experiencing the sense of hearing for the first time. Moreover, the patient is typically sedated from the implant procedure, which requires at least waiting until the patient can communicate in some way to perform the fitting.
Objective fitting methods have been developed for use with the patient sedated and possibly with children as well. Present objective fitting techniques measure physiological responses, such as the evoked compound action potential (ECAP), the middle ear reflex (MER), and the stapedius reflex (SR), to direct electrical stimulation of the intracochlear electrode. Cochlear stimulation systems that use objective fitting techniques typically include hardware and software components that provide the physician with control over the intensity of the electrical signals applied directly to the intracochlear electrodes. The electrical signals are typically biphasic, amplitude balanced pulses generated by an electrical signal source that is external to the cochlear stimulation system. The physiological responses are measured using either surface electrodes such as electroencephalographic (“EEG”) electrodes, cochlear stimulation system intracochlear electrodes themselves or implanted electrodes, and the objective is to measure the response of the auditory nervous system to the applied electrical signals.
Known objective fitting techniques suffer from various drawbacks. First, such methods typically require the use of special fitting components that are part of the cochlear stimulation systems. The special fitting components are often proprietary apparatuses and methods designed for exclusive use with particular cochlear stimulation systems. Second, the techniques require generating electrical stimulation to the intracochlear electrodes that bypass the operation mode of the sound processing system of the cochlear stimulation system. Third, the techniques generally proceed by setting a T level for some of the intracochlear electrodes one at a time. This is time-consuming when setting the T level for all of the intracochlear electrodes and not very accurate when extrapolating from the T levels determined for a set of intracochlear electrodes to determine T levels for the rest. Fourth, the fitting does not factor in sound at all. The physiological response is a response to an electrical signal, and not sounds.
Known objective fitting techniques have been determined to result in a poor correlation between the threshold levels indicated by psychophysical measurements, for example, and T and C levels. In many cases, techniques that rely on direct stimulation to measure ECAP, MER, SR, and other physiological responses typically result in an overstimulation of the intracochlear electrodes during operation. These known objective techniques work by measuring responses to stimulation of single electrodes. This approach does not factor in that the physiological responses are different when processing actual sounds that involve the cumulative effect of multiple electrodes.
In view of the above, there is a need for improved systems and methods for performing objective fitting of cochlear stimulation systems.